1. Field of the Invention
The present invention relates to an active type color EL (electroluminescence) display device in which an electroluminescence (EL) element is driven using a thin film transistor (TFT).
2. Description of Related Art
Practical use of organic EL elements in next generation display devices is greatly expected, because such displays can eliminate need for a back light as required in a liquid crystal display device for self-emission, can be optimally made thin, and can have an unlimited viewing angle.
Three methods have commonly been proposed for achieving color display in a display device comprising such an organic EL element.
In the first method, different emissive materials for each of the primary RGB colors are used in corresponding emissive layers to individually form discrete color pixels directly emitting respective RGB light rays. In another method, an emissive layer generates white luminescence which is then converted into three primary colors using color filters. A third method is based on conversion of light from a blue emissive layer into three primary colors using color conversion mediums (CCM). As light energy is lost in the second and third methods above due to the use of color filters or color conversion mediums, the first method is the most effective of these in this respect because a desired light ray is directly emitted.
Meanwhile, to drive an organic EL display device, two types of driving methods, a passive type using a passive matrix and an active type employing TFTs, are available. The circuit configuration shown in FIG. 1 may be used in an active display.
FIG. 1 illustrates a circuit configuration for a single pixel in such a display pixel. Each pixel comprises an organic EL element 20, a first TFT 21 for switching, in which a display signal DATA is applied to a drain and a scan signal SCAN is applied to a gate to switch the TFT on and off, a capacitor 22 which is charged by a display signal DATA applied when the TFT 21 is on and which holds a charge voltage Vh when the TFT 21 is off, a second TFT 23 in which a drain is connected to a drive source of a voltage VCOM, a source is connected to an anode of the organic EL element 20 and a hold voltage Vh is applied to a gate from the capacitor 22 to drive the organic EL element 20.
A scan signal SCAN rises to an H level during one horizontal scanning period (1H). When the TFT 21 is switched on, a display signal DATA is applied to one end of the capacitor 22, which is then charged by a voltage Vh corresponding to the display signal DATA. This voltage Vh remains held in the capacitor 22 for one vertical scanning period (1V) even after the signal SCAN becomes a low level to switch the TFT 21 off. Because the voltage Vh is supplied to the gate of the TFT 23, the EL element is controlled so as to emit light with a luminance in accordance with the voltage Vh.
The conventional configuration of such an active type EL display device for achieving color display by means of the above-mentioned first method will be now described.
FIG. 2 depicts a conceptual plan view showing a configuration of a related art device, and FIG. 3 is a cross section taken along line C—C in FIG. 2. Each of the drawings depicts three pixels.
In FIGS. 2 and 3, numeral 50 represents a drain line for supplying a display signal DATA, numeral 51 represents a drive source line for supplying a supply voltage VCOM, and numeral 52 represents a gate line for supplying a scan signal SCAN. Further, numerals 53, 54, and 55 designate features corresponding the first TFT 21, the capacitor 22, and the second TFT 23 in FIG. 1, respectively, and numeral 56 designates an anode of the EL element 20 which constitutes a pixel electrode. As shown, discrete anodes 56 are separately formed for each pixel on a planarization insulating film 60. A hole-transport layer 61, an emissive layer 62, an electron-transport layer 63, and a cathode 64 are sequentially laminated on the discrete anode 56, thereby forming an EL element. Holes injected from the anodes 56 and electrons injected from the cathodes 64 are recombined inside the emissive layer 62, which emits light in the direction of the transparent anodes toward outside, as shown by arrows in FIG. 3. Here, discrete hole-transport layers 61, discrete emissive layers 62 and discrete electron-transport layers 63 having substantially the same shape as the discrete anodes 56 are provided for respective pixels. Emissive materials which are different for each RGB are used in the corresponding emissive layers 62, and therefore light rays having respective RGB colors are emitted from respective EL elements. The cathode 64, which applies a common voltage to each pixel, extends over the pixels. Partitions 68 are interposed between adjoining emissive layers 62. Further, numerals 65, 66, and 67 designate a transparent glass substrate, a gate insulating film, and an interlayer insulating film, respectively.
However, the arrangement of the first TFT 53, the capacitor 54, the second TFT 55, and the anode 56 of the related examples do not take sufficient consideration of integration efficiency and therefore a more highly-integrated configuration is in demand.
Further, the color display device generally adopts a stripe arrangement as shown in FIG. 4A or a delta arrangement as shown in FIG. 4C as an arrangement for three primary colors of RGB. At the same time, it is necessary to use different luminescent materials for each of RGB such that discrete EL elements can directly emit light rays of respective RGB colors. Therefore, if the stripe arrangement shown in FIG. 4A is adopted, for example, a metal mask 70 shown in FIG. 4B may be used to form the luminescent layers as follows. First, a luminescent layer for R is formed by evaporating only an R color luminescent material onto the hole transport layer. Then, the metal mask 70 is displaced by a distance corresponding to one pixel in the horizontal direction to form a luminescent layer for G by evaporating only a G color luminescent materials on the hole transport layer. Finally, the metal mask 70 is further displaced by one pixel in the horizontal direction to form a luminescent layer for B by evaporating only a B color luminescent material. In the case of the delta arrangement shown in FIG. 4C, the luminescent layers can be similarly formed using the metal mask shown in FIG. 4D.
The above mentioned layer forming methods are disadvantageous in that, if dimensions of the metal mask or positioning at the time of metal mask displacement is not accurate, color purity is lowered because colors are mixed in adjoining pixels. Therefore, if a further highly integrated configuration is desired, a problem of positioning accuracy in the metal mask may occur.